In a variety of situations, it is desirable to have a radially stiff framework that can be collapsed to a very small diameter. The ability to expand five to ten-fold or greater has great utility in applications where space is at a premium. One use for an expandable space frame is in the construction of luminal stents, where the term xe2x80x9cstentxe2x80x9d is generically used to describe structural devices that support living tissues.
Stents are implanted in a body lumen for treating abnormal conditions. For example, these devices have found use in maintaining the patency of collapsing and partially occluded blood vessels, particularly to prevent acute closure and restenosis after a vessel has been enlarged by angioplasty. These devices have also been used to reinforce other body lumens, such as the urinary tract, the bile tract, the intestinal tract, and the tracheobronchial tree.
Conventional stents are cut from a tube or formed from a wire that has been bent back and forth in a zig-zag pattern and wound in a circumferential direction to form one or more loops of a pre-determined circumference. Typically, the stent is radially expandable from a collapsed condition. It is desirable to minimize the diameter of the collapsed stent so that it can be delivered as unobtrusively as possible through the vasculature. Once in position it is expanded to the predetermined size, to support and reinforce the lumen.
The stent is normally inserted in the collapsed condition by a catheter during intraluminal delivery to the repair site. Once properly located, the stent is removed from the catheter and radially expanded until its circumference firmly contacts the interior wall of the lumen. Usually the radial expansion is caused by the dilation of an angioplasty balloon placed axially within the stent. Alternatively, the stent may be made from a shape memory metal, whereby the stent will automatically assume its expanded circumference as its temperature increases upon implantation, or stents can be made that expand through spring action.
An important attribute of the stent is its ability to provide radial support. This capability is a concern not only where the stent is being used to maintain the patency of the lumen in which it is located, but also where the stent is being used in conjunction with a prosthetic graft to keep the graft open and to hold it at the location at which it is implanted.
The patent literature contains descriptions of many different stent designs. A few of the more recent patents include U.S. Pat. No. 5,702,419, xe2x80x9cExpandable, Intraluminal Stentxe2x80x9d; U.S. Pat. No. 5,707,388, xe2x80x9cHigh Hoop Strength Intraluminal Stentxe2x80x9d; U.S. Pat. No. 5,707,387, Flexible Stentxe2x80x9d; and U.S. Pat. No. 5,681,345, xe2x80x9cSleeve Carrying Stentxe2x80x9d; Palmaz, U.S. Pat. No. 5,102,417, xe2x80x9cExpandable intraluminal graft, and method and apparatus for implanting an expandable intraluminal graftxe2x80x9d; and Sigwart, U.S. Pat. No. 5,443,500, xe2x80x9cIntravascular stentxe2x80x9d.
Scientific reviews of stent design and function may be found in Wong et al. (1996) Catheterization and Cardiovascular Diagnosis 39:413-419; Sniderman (1996) Progress in Cardiovascular Diseases, vol. XXXIX:141-164. Fontaine and dos Passos (1997) Journal of Vascular and Interventional Radiology 8:107-111 present an example of pre-clinical analysis for a prototype stent. Hong et al. (1997) Coronary Artery Disease 8:45-48, describe pre-clinical use of a self-expanding nitinol stent.
Features desirable in a stent are reviewed by Palmaz (1992) Cardiovasc. Intervent. Radiol. 15:279-284. A highly desirable stent would combine a high expansion ratio with radial stiffness and lengthwise flexibility to facilitate insertion and/or conform to curves of the vessel once in place. The present invention provides this and other useful features.
An expandable space frame linking a plurality of flexible joints, e.g. springs; riveted or pinned joints; etc., joined through spacing arms to form a closed structure, e.g. circle, ellipse, rectangle, etc. is provided. The spacing arms are sterically offset, linking the bottom of one junction to the top of the next junction in an upwards stepwise fashion for a portion of the circle, and then reversing the steps to go down. The offset allows the frame to be collapsed with minimal steric hindrance, which permits a very high ratio of the expansion to compression diameters for the frame. The space frame forms the basis for different types of stents. A series of individual frames are linked to each other to form a stent. Alternatively, two or more frames are linked to longitudinal struts to form the support structure for a stent. The stents formed from the expandable space frame can be designed to have a number of additional features as set forth herein.